The present invention relates to a fluid-filled elastomeric suspension bushing.
Bushings typically used in automobile suspensions are expected to damp low frequency large displacement oscillations caused by road irregularities such as tar strips, pot holes and braking loads. Such bushings are also expected to contribute to high frequency acoustic isolation from relatively small displacements caused by road and tire noise.
The use of fluid-filled elastomeric bushings for use in automobile suspensions has previously been proposed. However, such proposals have entailed costly bushings, often of complex design, having shorter useful lives than conventional bushings and entailing more complex manufacturing procedures. These are described in more detail below.
There are bushings manufactured by the assignee of the present invention in which an elastomeric cylindrical member with a central axial bore is axially inserted into a metal outer tube. An inner metal tube may then be inserted into the elastomeric member bore. The elastomeric member has an initial outside diameter which is larger than the inside diameter of the outer tube and an initial length which is shorter than the length of that tube. The initial shape and size of the elastomeric member before it is inserted into the outer tube and before the inner tube is inserted into the elastomeric member bore determine the final shape and size of the elastomeric member. The elastomer member is inserted into the outer tube by progressively forcing the elastomeric member through a funnel-like device, the output end of which temporarily communicates with the interior of the outer tube.
Axial movement of the elastomeric member through the funnel progressively decreases the member diameter and increases its length until the diametrically decreased and lengthened elastomeric member resides within the outer tube. The inner metal tube may then be subsequently inserted into the central bore of the elastomeric member. The above described bushings and other prior art non-fluid bushings are generally only partially effective in damping either low frequency large displacement oscillations or isolating high frequency small displacement oscillations as illustrated by the dashed line curves in FIGS. 8 and 9, which curves represent the same parameters in both Figures. As seen, the damping characteristics increase gradually with frequency, but do not address resonant frequencies of particular concern such as low frequency oscillations at frequency f.sub.1, FIG. 8, or high frequency oscillations in the region of frequency f.sub.2, FIG. 9. Low frequencies in this application are, for example about 20 Hz or less and refer to vehicle shake and high frequencies are, for example, above 50 Hz.
To meet this problem, there are available tunable bushings which use an incompressible fluid contained in a plurality of pockets formed in the elastomeric material which are in communication with each other via a connecting channel. A ring over lies the elastomeric bushing and has openings corresponding to the pockets and a depression corresponding to the channel communicating with the pockets.
The problem with this arrangement is that the elastomeric bushing needs to be bonded to the inner tube member in a mold apparatus. The bonding is provided by coating the inner tube with a bond agent prior to insertion into the mold apparatus which injection molds, i.e., vulcanizes, the rubber elastomeric member to the inner tube. A bonding agent is costly and environmentally undesirable because of the undesirable chemicals employed.
A further problem is complexity of manufacture. After the above vulcanizing step, the elastomer-tube assembly then needs to be attached to the ring and to the outer tube. The ring is placed within the outer tube with clearance therebetween and they are then placed over the elastomer member with clearance between the ring and outer tube and the elastomeric member. The outer tube is then radially compressed with a die about 5-10% to secure the ring and outer tube to the elastomer member. The assembly during the die compressing step may be immersed in the desired damping fluid to fill the pockets and channel with the fluid or the fluid may be injected into the pockets and channel later. The use of the bonding agent together with the relatively complex assembly result in a costly device. A further problem with this assembly is durability. The life of such a bushing is not as long as needed for automotive applications, thus requiring premature bushing replacement. This is undesirable.
The present inventors recognize a need for a low cost tunable fluid-filled hydroelastic bushing which effectively selectively damps or isolates oscillations of a given frequency and which is inexpensive, simple in design, durable and expedient to manufacture.